Thermal, physical vapor deposition is a well known technique for the coating of a substrate with a material which is held in a container or in a housing, the deposition source, and heated to the point of vaporization, the vapor moving out of the deposition source and condensing on a substrate to be coated. Most often this process is carried out with both the deposition source holding the material to be vaporized and the substrate to be coated contained in a vessel which is evacuated to a level of pressure in a range of from 10.sup.-7 to 10.sup.-2 Torr. Such reduced pressure is useful in avoiding unwanted reactions between either the materials of which the source is constructed or the deposition materials contained in the deposition source with the atmosphere in the vessel as the temperature of the deposition source is increased to the point where the deposition materials vaporize.
Typically, the deposition source is made from an electrically resistive material whose temperature will increase when an electrical current is passed through the container walls. The deposition material inside is then heated by radiation from the walls and by conduction from contact with the walls. Typically, the container is shaped like a box, with an aperture to allow escape (efflux) of the vapor in the direction of the substrate. However, other methods of heating the walls have been used, including radiation from coils surrounding the container, induction heating of the container with suitable coils, electron impact, and irradiation from an optical source.
If the container or housing is fabricated from a material in such a way as to be liquid-tight, then there is no concern whether vaporization is by sublimation from the solid or evaporation from a liquid formed as the solid first melts. In any case, it is well known that the container can be lined with a high-temperature-compatible, liquid-tight material, such as quartz, a ceramic, a shaped carbon material, or the like and that such liner be heated by a surrounding resistive heater.
Thermal, physical vapor deposition sources have been used to vaporize and deposit onto a substrate layers comprised of a wide range of materials, for example low temperature organics, metals, or fairly high temperature inorganic compounds. In the case of organic layer deposition, the starting material is typically a powder. It has been recognized that such organic powders present a number of challenges to this type of thermal vaporization coating. First, many of the organics are relatively complex compounds (high molecular weight), with relatively weak bonding, so that care must be taken to avoid decomposition during the vaporization process. Next, the powder form can give rise to particles of unvaporized electroluminescent material leaving the deposition source with the vapor and being deposited as undesirable lumps on the substrate. Such lumps are also commonly referred to as particulates or particulate inclusions in a layer formed on a substrate. This is additionally exacerbated in that the powder form also has a very large surface area which may hold absorbed or adsorbed water or volatile organics which can be released during heating and can cause eruptions of gas and particulates to be thrown outward from the deposition source toward the substrate. Similar considerations pertain to materials which melt before vaporization (evaporation) and can result in droplets being ejected to the substrate surface.
There are many applications where these unwanted particulates or droplets will result in unacceptable defects in a product, particularly in electronic or optical applications where dark spots may result in images or shorts or opens may result in electronic device failures.
There has been much effort spent in devising organic deposition sources designed to more uniformly heat such an organic powder charge and to also prevent the bursts of particulates or droplets from reaching the substrate. Numerous designs of complicated baffling structures between the source material and the exit opening have been suggested to try to assure only vapor exits. See for examples U.S. Pat. Nos. 3,466,424 and 4,401,052. There are also various elaborate container features added interior to the source material container seeking to achieve maximum contact area between the organic vapor deposition material and hot members of the container. See for examples U.S. Pat. Nos. 2,447,789 and 3,271,562.